These are common parasites of the tissues and digestive system of all classes of vertebrates. Some are serious pathogens.
Life cycles may be direct or indirect.
Adults are usually of moderate size, 10-20 mm long. There are usually 3 or 6 small lips. The esophagus usually varies in diameter along its length, although not so pronounced as in the Rhabditida. A distinctive feature of the group is the well developed copulatory bursa possessed by the male, with its long, muscular supporting rays.